1. Field of the Invention
This invention relates to a composition, an article and a method. More specifically, the compositions embraced within the scope of this invention are highly efficient photogenerator materials and are thus suitable for use in electrophotographic imaging members and methods.
2. Description of the Prior Art
The formation and development of images on the imaging surfaces of photoconductive materials by electrostatic means is well-known. The best known of the commercial processes, more commonly known as xerography, involves forming a latent electrostatic image on the imaging surface of an imaging member by first uniformly electrostatically charging the surface of the imaging layer in the dark and then exposing this electrostatically charged surface to a light and shadow image. The light struck areas of the imaging layer are thus rendered relatively conductive and the electrostatic charge selectively dissipated in these irradiated areas. After the photoconductor is exposed, the latent electrostatic image on this image bearing surface is rendered visible by development with a finely divided colored electroscopic material, known in the art as "toner". This toner will be principally attracted to those areas on the image bearing surface which retain the electrostatic charge and thus form a visible powder image.
The developed image can then be read or permanently affixed to the photoconductor where the imaging layer is not to be reused. This latter practice is usually followed with respect to the binder-type photoconductive films (e.g. zinc oxide/insulating resin binder) where the photoconductive imaging layer is also an integral part of the finished copy, U.S. Pat. Nos. 3,121,006 and 3,121,007.
In so-called "plain paper" copying systems, the latent image can be developed on the imaging surface of a reusable photoconductor or transferred to another surface, such as a sheet of paper, and thereafter developed. When the latent image is developed on the imaging surface of a reusable photoconductor, it is subsequently transferred to another substrate and then permanently affixed thereto. Any one of a variety of well-known techniques can be used to permanently affix the toner image to the copy sheet, including overcoating with transparent films, and solvent or thermal fusion of the toner particles to the supportive substrate.
In the above "plain paper" copying systems, the materials used in the photoconductive layer should preferably be capable of rapid switching from insulating to conductive to insulating state in order to permit cyclic use of the imaging surface. The failure of a material to return to its relatively insulating state prior to the succeeding charging/imaging sequence will result in an increase in the rate of dark decay of the photoconductor. The phenomenon, commonly referred to in the art as "fatigue" has in the past been avoided by the selection of photoconductive materials possessing rapid switching capacity. Typical of the materials suitable for use in such a rapidly cycling imaging system include anthracene, sulfur, selenium and mixtures thereof (U.S. Pat. No. 2,297,691); selenium being preferred because of its superior photosensitivity.
In addition to the anthracene, other organic photoconductive materials, most notably, poly(N-vinylcarbazole), have been the focus of increasing interest in electrophotography, U.S. Pat. No. 3,037,861. Until recently, neither of these organic materials have received serious consideration as an alternative to such inorganic photoconductors as selenium, due to fabrication difficulties and/or to a relative lack of speed and photosensitivity within the visible band of the electromagnetic spectrum. The recent discovery that high loadings of 2,4,7-trinitro-9-fluorenone in polyvinylcarbazoles dramatically improves the photoresponsiveness of these polymers has led to a resurgence in interest in organic photoconductive materials, U.S. Pat. No. 3,484,237. Unfortunately, the inclusion of high loadings of such activators can and usually does result in phase separation of the various materials within such a composition. Thus, there will occur within these compositions regions having an excess of activator, regions deficient in activator and regions having the proper stoichiometric relation of activator to photoconductor. The maximum amount of activator that may be added to most polymeric photoconductive materials without occasioning such phase separation generally will not exceed in excess of about 6 to about 8 weight percent.
One method suggested for avoiding the problems inherent in the use of such activators in conjunction with polymeric photoconductors, is the direct incorporation of the activators into the polymeric backbone of the photoconductor, U.S. Pat. No. 3,418,116. In this patent is disclosed the copolymerization of a vinyl monomer having an aromatic and/or heterocyclic substituent capable of an electron donor function with a vinyl monomer having an aromatic and/or heterocyclic substituent capable of an electronic acceptor function. The spatial constraint placed upon these centers of differing electron density favors their charge transfer interaction upon the photoexcitation of such a composition. These so-called "intramolecular" charge transfer complexes, more accurately designated "intrachain" charge transfer complexes, are believed to function substantially the same as charge transfer complexes formed between small activator molecules and a photoconductive polymer. The fact that the electron donor function and an electron acceptor function are on a common polymeric backbone does not apparently change the .pi. -- .pi. charge transfer interaction, but merely increases the probability of it occuring. Unfortunately, the preparation of such polymers from vinyl monomers having electron donor centers and vinyl monomers having electron acceptor centers is often beset with difficulty.
The preparation of non-polymeric photoconductive tricyanovinyl compounds, wherein an electron rich center and an electron deficient center are contained within a common molecule, is disclosed in U.S. Pat. No. 3,721,552 (corresponding Australian patent application Ser. No. 36,760/68, published Oct. 10, 1969). Patentee discloses the preparation of photoconductive "binder" layers by dispersing from about 10 to about 90 parts by weight of his novel tricyanovinyl compounds in about 90 to about 10 parts by weight resin binder. The binder resins which can be used in preparation of the photoconductive insulating layer must have an electrical volume resistivity in excess of 10.sup.8 ohm - cm. Virtually any of the binders traditionally employed in preparation of electrophotographic imaging members are reportedly suitable in the preparation of these binder layers. Insofar as the preferred weight ratio of photoconductive particles to binder resin is 1:1, it is apparent that Patentee does not appreciate that sufficiently lower loadings of such compounds in a charge transport matrix can produce results equivalent to his preferred composition. By minimizing the amount of photoconductive compound needed to achieve satisfactory photoresponse, the inherent physical properties of the film forming binder resin are preserved (e.g. flexibility, adhesion, and free surface energy).
It is the principal object of this invention to provide a novel class of photogenerator compounds which are suitable for use in photoconductive compositions.
It is another object of this invention to provide a photogenerator compound having a high extinction coefficient.
It is yet another object of this invention to provide a photogenerator compound wherein charge transfer interaction between a donor and acceptor site occur independent of the relative concentration of the photogenerator compounds in the resin.
It is yet a further object of this invention to provide a photoconductive composition having broad spectral response in the visible region of the electromagnetic spectrum.
Further objects of this invention include providing imaging members wherein the imaging layer is prepared from the above composition and the use of said imaging members in an imaging method.